Beams, trusses, joists, and columns are the typical structural members that support the weight or loads of structures, including buildings and bridges. Structural members may be manufactured from a variety of materials, including steel, concrete, and wood, according to the structure design, environment, and cost.
Wood structural members are now typically manufactured from multiple wood segments that are bonded together, such as in glue-laminated members, laminated veneer lumber, parallel strand lumber and I-beams. These manufactured wood structural members have replaced sawn lumber or timbers because the former have higher design limits resulting from better inspection and manufacturing controls. Wood is a desirable material for use in many structural members because of its various characteristics, including strength for a given weight, appearance, cyclic load response, and fire resistance.
In any application, a load subjects a structural member to both compressive and tensile stresses, which correspond to the respective compacting and elongating forces induced by the load on opposite sides of the member. By convention, a neutral plane or axis extends between the portions of the member under compression and tension. The structural member must be capable of bearing the compressive and tensile stresses without excessive strain and particularly without ultimately failing.
Reinforcement of wood structural members in regions subjected to tensile stresses are known. For example, U.S. Pat. No. 5,026,593 of O'Brien describes the use of a thin flat aluminum strip to reinforce a laminated beam. The use of a synthetic tension reinforcement having multiple aramid fiber strands held within a resin matrix adhered to at least one of the wood segments in the tension portion of the structural member is described by the inventor of the present application in "Reinforced Glued-Laminated Wood Beams" presented at the 1988 International Conference on Timber Engineering. However, wood structural members also include regions subjected to compressive stresses, and the reinforcements suitable for tensile reinforcement are typically not suitable for compressive reinforcement.
Many glue laminated structural wood members are manufactured according to manufacturing standards 117-93 of the American Institute of Timber Construction (AITC) of Englewood, Colo. These manufacturing standards specify, for example, a visual wood knot grade in which knots in the wood laminae in the tension zone, defined as the 10 percent of the depth a beam under greatest tension, have diameters of no more than 25 percent of the widths of the wood laminae. Aside from such visual grade specifications, however, the positions of wood laminae of varying stiffness and strength are allowed to be random in the conventional manufacture of glue laminated structural wood members.
The AITC manufacturing standards and the conventional manufacturing practices provide glue laminated wood members that are of generally high quality and have generally high design limits. However, limited availability of affordable higher quality wood can increase the cost or reduce the quality of glue laminated wood members. In addition, relatively inexpensive alternative structural materials such as steel and concrete require that the quality and design capabilities of glue laminated structural wood members be improved to maintain or improve competitiveness.